1. Field of the Invention
The present invention concerns a non-aqueous electrolyte secondary battery charged at a high voltage using a positive electrode active material that is charged at a charge termination potential ranging from 4.4 to 4.6 V based on lithium and more specifically, to a non-aqueous electrolyte secondary battery excellent in cycle characteristics charged at a charge termination potential ranging from 4.4 to 4.6 V based on lithium using a positive electrode active material comprising a mixture of a lithium-cobalt composite oxide containing at least both of zirconium and magnesium in LiCoO2, and a lithium-manganese-nickel composite oxide having a layered structure and containing at least both of manganese and nickel.
2. Description of the Related Art
Along with the rapid and widespread use of portable electronic equipments, specifications required for batteries used therein have become more and more stringent, and those that are small in size, thinly shaped, yet have high capacity, and exhibit excellent cycle characteristics and stable performance have become particularly desirable. In the field of secondary batteries, non-aqueous electrolyte lithium secondary batteries have been noted for higher energy density compared with batteries of other types such that the market share of non-aqueous lithium electrolyte secondary batteries has remarkably grown.
In equipment where the non-aqueous electrolyte secondary battery of the above-described type is used, the battery is square-shaped as it is formed by disposing the power generation elements in a square outer casing and the space for containing the battery is often square-shaped in the form of a flat box. An example of such a square-shaped non-aqueous electrolyte secondary battery is described hereafter with reference to FIG. 1.
FIG. 1 is a perspective view along the longitudinal direction of a square-shaped non-aqueous electrolyte secondary battery of prior art. In a non-aqueous electrolyte secondary battery 10, a flat spiral electrode body 14, in which a positive electrode plate 11 and a negative electrode plate 12 are wound while interposing a separator 13 therebetween is contained inside a square battery outer case 15 which is sealed by a sealing plate 16.
The spiral electrode body 14 is wound such that the positive electrode plate 11 is exposed while being positioned at the outermost periphery of the spiral electrode body 14, at which it is in direct contact with and electrically connected to the inner surface of the battery outer case 15 which also serves as a positive electrode terminal. Further, the negative electrode plate 12 is formed at the center of the sealing plate 16 and connected electrically to a negative electrode terminal 18 through a collector 19, where the negative electrode terminal 18 is attached through an insulator 17 that also serves as a negative electrode terminal.
Then, since the battery outer case 15 is electrically connected to the positive electrode plate 11 to prevent the occurrence of short circuit between the negative electrode plate 12 and the battery outer case 15, an insulation spacer 20 is inserted between the upper end of the spiral electrode body 14 and the sealing plate 16, thereby electrically insulating the negative electrode plate 12 and the battery outer case 15 from each other.
The square-shaped non-aqueous electrolyte secondary battery is produced by inserting the spiral electrode body 14 into the battery outer case 15, laser welding the sealing plate 16 to the opening of the battery outer case 15, then injecting a non-aqueous electrolyte from an electrolyte injection port 21 and tightly closing the electrolyte injection port 21. Such a square-shaped non-aqueous electrolyte secondary battery provides excellent effects by taking up less space during use and having high performance and reliability.
The negative electrode active material used in the above-described non-aqueous electrolyte secondary consists of carbonaceous materials such as graphite and amorphous carbon which are generally used because of their excellent properties of initial efficiency and high safety by inhibiting the growth of dendrites, and have satisfactory potential flatness as well as high density while having a discharge potential comparable to that of a lithium metal or lithium alloy.
Further, carbonates, lactones, ethers, esters, etc. are used singly or in combination as non-aqueous solvent for the non-aqueous electrolyte. In particular, carbonates having high dielectric constant and high ionic conductivity are often used to produce the non-aqueous electrolyte.
Since the non-aqueous electrolyte of the non-aqueous electrolyte secondary battery may sometimes overheat due to over charging or short circuit as to evolve gases leading it to swell, ignite or explode, various additives are used together to ensure safety performance. For example, JP-A No. 2004-214139 (claims; Patent Document 1) discloses an example of using a cyclic carbonate ester of an unsaturated hydrocarbon as non-aqueous solvent and using at least one member selected from the group consisting of cyclohexyl benzene and derivatives thereof and at least one member selected from the group consisting of vinylene carbonate, vinylethylene carbonate and derivatives thereof as an additive for ensuring safety of the battery upon overcharging.
Further, JP-A No. 2004-349131 (claims; Patent Document 2) discloses a non-aqueous electrode to which an aromatic compound represented by the following chemical formula (I) is added:
where each of R1 and R2 independently represents an alkyl group which may have a substituent, or, R1 and R2 may be joined to form a hydrocarbon ring which may have a substituent, where the ring A may have a substituent and at least one carbon atom adjacent to carbon atoms to which R1R2CH— is bonded has a substituent).
Further, JP-A No. 10-275632 (claims; Patent Document 3) discloses a non-aqueous electrolyte containing a non-ionic aromatic compound having an alkyl group.
On the other hand, it has been publicly known that a 4 V class non-aqueous electrolyte secondary battery of high energy density can be obtained by using the combination of a positive electrode active material comprising a lithium composite oxide such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiFeO2, etc. and a negative electrode comprising a carbon material. Among these lithium composite oxides, LiCoO2 has often been used because the batteries exhibit various excellent characteristics compared to others. However, since cobalt is expensive and natural resources are rather limited, efforts have been made to determine whether other transition elements which may yield battery characteristics that are equal to or even exceed those obtained by using cobalt may be substituted, as demand continues to grow for non-aqueous electrolyte secondary batteries with better performance and longer life.
For example, a method of adding foreign elements such as Zr or Mg to LiCoO2 for the purpose of improving the characteristics of a non-aqueous electrolyte secondary battery using LiCoO2 as positive electrode active material has been disclosed in JP-A No. H4-319260 (claims, and columns [0006], [0008] to [0011]; Patent Document 4) and JP-A No. 2004-299975 (claims, and columns [0006] to 00008); Patent Document 5). Patent Document 4 discloses a non-aqueous electrolyte secondary battery capable of generating a high voltage and showing excellent charge/discharge and shelf life characteristics by adding zirconium to LiCoO2 as positive electrode active material. When zirconium is added to LiCoO2 as positive electrode active material, the surface of LiCoO2 particles is stabilized by being covered with zirconium oxide (ZrO2) or composite oxide of lithium and zirconium (Li2ZrO3) and, as a result, a positive electrode active material showing excellent cycle and shelf life characteristics can be obtained without causing decomposing reaction in the electrolyte or destruction of crystals even at high potential. Such effect cannot be obtained by merely mixing LiCoO2 after burning with zirconium or zirconium compound but is obtained by adding zirconium to a mixture of lithium salt and the cobalt compound and burning them. Patent Document 5 also discloses that by adding not only zirconium (Zr) but also at least one other member such as titanium (Ti) and fluorine (F) as foreign elements to LiCoO2 as positive electrode active material, the load and cycle characteristics of the non-aqueous electrolyte lithium secondary battery can be improved.
Further, JP-A No. 2002-042813 (claims and columns [0011]-[0016]; Patent Document 6) discloses a non-aqueous electrolyte secondary battery capable of generating a high voltage and showing excellent charge/discharge characteristics and high capacity by using an Li-transition metal composite oxide having a layered structure in which the compositional ratio between Ni and Mn is substantially equal, producing a voltage of about 4 V equal to that produced by LiCoO2, while JP-A No. 2004-296098 (claims; Patent Document 7) disclose that by using a positive electrode active material containing at least Ni and Mn as transition metals and a lithium-transition metal composite oxide having a layered structure further containing fluorine, a non-aqueous electrolyte secondary battery capable of being charged at a charging voltage of 4.4 V or higher and excellent in thermal stability can be obtained, by combining the positive electrode active material with the negative electrode containing a carbon material as negative electrode active material. Further, Electrochemical and Solid-State Letters 4 (12) A200-A203 (2001) (Non-Patent Document 1) show a battery with specifically high thermal stability even in a charged state (high oxidized state) when, from among the lithium transition metal composite oxides above-mentioned, that which contains Ni, Co and Mn, where the material represented by the chemical formula: LiMnxNixCo(1-2x)O2 is used, in which the compositional ratio between Mn and Ni is equal.
The charging voltage of the current non-aqueous electrolyte secondary battery using a lithium-containing transition metal oxide such as lithium cobalt oxide (LiCoO2) as positive electrode active material and using a carbon material as negative electrode active material, when combined with the negative electrode active material of carbon material such as graphite, ranges from 4.1 to 4.2 V (potential of positive electrode active material is 4.2 to 4.3 V based on lithium). Under such charging condition, only about 50 to 60% of the capacity of the positive electrode is utilized based on theoretical capacity. Accordingly, if the charging voltage can be increased, as much as 70% of the capacity of the positive electrode relative to theoretical capacity can be utilized, or higher, thereby increasing the capacity and energy density of the battery.
In view of the state of developments involving the nature of positive electrode active material for use in the non-aqueous electrolyte secondary battery described above, the present applicant has made various studies to formulate a positive electrode active material which would render a non-aqueous electrolyte secondary battery capable of attaining high charging voltage more stably and in the process has developed a novel non-aqueous electrolyte secondary battery using a mixture of lithium cobalt oxide to which foreign elements are added and layered lithium manganate-nickelate as positive electrode active material, and which invention is now subject of Japanese Patent Applications Nos. 2004-094475 and 2004-320394 (hereinafter collectively referred to as “Prior Applications”).
As disclosed in the prior applications, the structural stability of the positive electrode active material of the non-aqueous electrolyte secondary battery is improved at high voltage (about 4.5V) by adding at least Zr and Mg as foreign elements to lithium cobalt oxide and ensuring safety by mixing it with layered lithium manganese nickel oxide of high thermal stability at high voltage. The combination of the positive electrode using such positive electrode active material and the negative electrode comprising negative electrode active material formed of carbon material can yield a non-aqueous electrolyte secondary battery capable of being charged at a high charging voltage of 4.3 V or higher and 4.5 V or lower (4.4V or higher and 4.6 V or lower of charge termination potential based on lithium).